The present invention relates generally to protecting driver circuits and ballasts from input power surges. More particularly, this invention pertains to protecting driver circuit and ballast inverters during an input power surge.
Lightning causes high-voltage surges in power supplies to electronics. To meet the standard for outdoor surge protection, a circuit must survive a 6 KV surge combo-wave. Referring to prior art FIG. 1, most driver circuits or ballasts include a half bridge based inverter or converter. The driver circuit 100 receives power from a power supply 102 and provides power to a load 104. The driver circuit 100 includes an input stage 116, a surge protector 114, a voltage regulator 108, a controller 106, and an output stage 118. The surge protector 114 limits the voltage from the power source 102 to the input stage 116. The input stage 116 includes a full wave rectifier, and a smoothing capacitor C1. Four diodes (i.e., diodes D1-D4) form a full wave rectifier operable to receive alternating current (AC) power from the power supply 102 and provide a direct current (DC) power rail 112. The smoothing capacitor C1 is an electrolytic capacitor the buffers the DC power rail. The voltage regulator 108 and a second capacitor C2 provide a bias voltage VCC to the controller 106. The output stage 118 is a half bridge inverter including a first switch Q1 and a second switch Q2. The controller 106 drives the first switch Q1 and the second switch Q2 such that the output stage 118 provides AC power to the load 104 from the DC power rail 112. The controller 106 provides drive signals to the first switch Q1 and the second switch Q2 so long as the controller 106 is receiving the bias voltage VCC from the oldest regulator 108 and a disable pin of the controller 106 is receiving the bias voltage VCC (i.e., not grounded).
When a high-voltage surge appears at the power source 102, even with the surge protector 114, high-voltage is created across the smoothing capacitor C1 (i.e., at the DC power rail 112). Without any further protection, the first switch Q1 and the second switch Q2 will not survive the surge because the first switch Q1 and the second switch Q2 are designed to meet the steady-state voltage requirement. That is, if the DC power rail operates at approximately 470 V DC, a 600 V rated first switch Q1 and second switch Q2 will be used. If the DC power rail 112 exceeds 600 V during the surge (which frequently happens), the voltage rating of the first switch Q1 and the second switch Q2 will be exceeded, causing failure of the output stage 118 and thus the driver circuit 100.